n-Butane isomerization was investigated at the supercritical state in a flow type reactor. Sulfated zirconia (SZ), heteropolyacids (HPA) of the Keggin type H3PW12O40 (HPWO), H4SiW12O40 (HSiWO) supported on titania, H-mordenite (HM) were studied as catalysts at 488, 533 and 573 K and 6.1, 11.0 and 13.8 MPa, respectively. Gas-phase isomerization was carried out on the same catalysts for comparison. Rapid deactivation of the studied catalysts was observed in the latter conditions. Conducting the reaction of n-butane isomerization in the supercritical phase resulted in the stable activity. No poisoning of catalysts was observed. It was shown that even the catalyst aged in the gas-phase isomerization can be reactivated by the supercritical media and the catalyst performance can attain the initial value of 20-25% conversion, when the n-butane density is in the vicinity of its critical value or higher. The selectivity for i-butane reached 80% on SZ and supported HSiWO/TiO2 catalysts in supercritical n-butane. At the same time, the i-butane selectivity on HM zeolite did not exceed 40% at 25% conversion. Cracking side reactions predominated at an elevated temperature (573 K) on the zeolite catalyst yielding the following by-products: propane, n-pentane, i-pentane and traces of ethane and hexane. (C) 2004 Elsevier B.V. All rights reserved.